I think Matthias goes well with Marco and Miles and its similar to Matthew on your list. I think you should stick with the M's that way your child doesn't feel "left out". For a girl I'd suggest Melissa or Molly

I don't think it's necessary to stick to an M theme. I'd say the 3rd child is where you have to decide if you want a theme or not though. If you go with an M name for #3 and then decide on a 4th child, it would be a little weird not to give #4 an M name too. But 2 M names and then something different looks more like a coincidence to me.